The current prior art still comprises mostly stabilization systems aimed at an osseous fusion (stiffening) of affected vertebrae. Elastic systems which merely support and stabilize segments of the vertebral column, but are not supposed to fuse it, have appeared only sporadically so far. These recent elastic systems still involve considerable implant time and expenditure. For reasons of production costs, operating time, and security in handling, future vertebral column stabilization systems should be as simple as possible.
The advantages of elastic stabilizations are becoming increasingly well known, above all by young patients, so that various inventors have developed and disclosed such systems. However, these inventions involve drawbacks which will be explained with the aid of the following examples.
Although the invention according to patent EP 0498 709 B1 to Graf intends to stabilize elastically, it has the drawback that the system works only in flexion (tension) but not in extension (compression). The stabilization is usually produced by means of at least two individual, mutually offset textile loops.
The invention according to patent application WO 93/20771 of Mazel also intends to connect vertebrae by means of pairs of flexible longitudinal rods. However, this invention has the drawbacks that the longitudinal rods have hardly any resistance to buckling and therefore can hardly transmit compressive forces, that the flexibility of the rods disposed in pairs cannot be the same in all directions, and that the transmission of force to the bone screws must take place by means of numerous components.
The invention according to patent EP 0516 567 B1 to Navas proposes the insertion of shock absorbers between the vertebrae. This invention has the drawbacks, however, that these shock absorbers are not longitudinally adjustable and that the plastic material is passed through a narrow place (neck), which results in a considerable decrease in strength.
The invention according to U.S. Pat. No. 5,282,863 A to Burton also intends to stabilize flexibly. However, this invention has the drawbacks that the system is too wide and can be put in place posteriorly only if the pedicles are removed, that it can be used for only one segment, that it cannot, for example, be passed through three screws lying one behind the other and offset, that the bore in the connecting element results in considerable weakening, and that the oval cross-section of the connecting element has only minimum shear strength and minimum resistance to buckling in the anterior/posterior direction.
Furthermore, for fastening the connecting element to the pedicle screw, a locking cap must be used, which is a disadvantage during the operation.
The invention according to patent application EP 0 667 127 A1 of Sanders seeks to achieve a certain elasticity by means of a metallic connection in that the form of the connection part allows local bending. However, this invention has the drawback that the connecting parts are not longitudinally adjustable and cannot be inserted multisegmentally.
The invention according to patent EP 0669 109 B1 to Baumgartner et al. likewise intends to stabilize adjacent vertebral segments elastically by using a cord for tensile forces and a plastic cushion for compressive forces. This invention has the drawbacks, however, that the system contains an expensive cord band, any desired cushion heights can be achieved only by means of a plurality of standard cushions, variation of the preload on the cord leads to conditions not reproducible biomechanically, and implantation is relatively expensive and takes a long time.